1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device. More particularly, the present invention relates to a method for fabricating a semiconductor device in which a semiconductor chip is processed after the semiconductor chip is mounted on a substrate.
2. Description of the Related Art
One example of a conventional semiconductor device which is structured by mounting a plurality of semiconductor chips on a substrate is described in Photonics Technology Letters, vol. 7, No. 4, pp. 360-362 (1995). According to this conventional technique, several tens of GaAs multi quantum-well light modulators (hereinafter, simply referred to as the "light modulator(s)") are integrated in a hybrid manner on a Si substrate having thereon a previously-formed CMOS transistor. The light modulator is supplied with a bias voltage by the CMOS transistor on the Si substrate, so that a reflectivity changes with respect to input light from the back side of the light modulator. As a result, the intensity of the reflected light is changed. In other words, the light modulator modulates the intensity of the reflected light by modulating the bias voltage, thereby performing signal processing.
FIGS. 18A-18D are cross-sectional views showing steps of a conventional method for fabricating a semiconductor device.
First, as shown in FIG. 18A, Pb/Sn solders 1803 are provided in a predetermined pattern on a CMOS transistor 1805 formed on a Si substrate (not shown) and on a light modulator 1801 formed on a GaAs substrate 1802, respectively.
Next, as shown in FIG. 18B, the Pb/Sn solders 1803 are fused so as to be bonded to each other. Thereafter, as shown in FIG. 18C, epoxy resin 1806 is heated so as to be at about 100.degree. C., thereby reducing its viscosity, and injected between the CMOS transistor 1805 and the light modulator 1801.
Then, as shown in FIG. 18D, the GaAs substrate 1802 is removed by wet etching. Upon performing the wet etching, the epoxy resin 1806 between the CMOS transistor 1805 and the light modulator 1801 serves as a layer for protecting the surface of the light modulator 1801 from an etchant. Finally, an antireflection film (not shown) is deposited on the surface of the light modulator 1801 after the GaAs substrate 1802 has been removed. Thus, the fabrication steps are completed.
According to the above-described conventional technique, the gap between the light modulator 1801 and the CMOS transistor 1805 is filled with the epoxy resin 1806 having a reduced viscosity. For that purpose, it is necessary to precisely control the heating temperature.
Furthermore, in the above conventional technique, voids (i,e., very small bubbles) may be generated in the epoxy resin 1806. Thus, upon performing the wet etching, the surface of the light modulator 1801 may be damaged.